Electron beam apparatus



Sept. 21, 194s.

R. H. vARlAN 2,449,569

Y ELE'CTRCN BEAM vAPPARATUS Filed Jan. 18, 194s INVENTOR 'RUSSELL H. VARIAN Patented Sept. 21, 1948 LEc'rRoN BEAM APPARATUS `Russell H. Varian,` Wantagh, N. Y., assignor to` The Sperry Corporation, a corporation of Dela- Ware Application January 1s, 1943, serial No. 472,783

1a claims. (ci. 315-5) The present invention is related to the art including electron beam" devices and especially to` Velocity modulation electron discharge devices ofthe type disclosed in Varian Patent No. 2,242,- 275, issued May 20, 1941. In such devices, and in many other types of electron discharge devices utilizing relatively long electron streams, difficulty is often experienced in attaining satisfactory operationl due to the dispersion of the electron beam caused by the mutual repelling action of the electrons comprising the beam.`

In the past, attempts to overcome thisdisa'dvantage were made by relying on the ionizing of a residual and extremely low-pressure gas within the evacuated envelope of the device, whereby the positive ions serve to neutralize the dispersing eiect of the electron stream. This action has been found to be effective with respect to` relatively low velocity electron beams and ordinary degrees of evacuation. However, for higherzpower and longer life tubes, it is necessary to` use better evacuation and higher velocity beams.` Under these conditions the dispersion of the beam is no longer prevented. VInvestigation has shownthat the `failure to maintain the beam denition is caused by the leakage of the cathode electrostatic eld beyond the usual accelerating grid, through the grid openings; Plottingof the electrostatic field shows that near the grid, and on the side thereof remote from the cathode, an electric iield gradient exists which attracts positive ions toward the cathode'These ions therefore move through the grid toward the cathode, leaving-the electron beam unneutralized at this point. The unneutralized electron beam forms aresultant negative space charge region which draws positive ions from regions farther from the grid, thus further extending the negative space charge and extending the region in whichl this drainage effect is evidenced. `In this way, the positive ionsV are prevented from maintaining the beam sharply defined. 1 s

According to the `present invention, this effect may bey eliminated by interposing 1an` ion `suppressor or shielding grid immediately beyond the rst anode or accelerating grid; that is, on the side of such accelerating grid remote from the cathode. By impressing a positiveor zero potential on this ion-suppressor `grid relative 'to the anode or accelerating grid, no positive ion formed in the beam'beyond thision-suppressor grid can reach the ion-suppressor electrode and, accordingly, theidrainageof positiveions from ythe beam inthis region is prevented. y

, 2 i i Accordingly, it is an object of the present invention to provide improved electron discharge tube apparatus of the electron beam type in which the dispersion of the electron beamfis greatly reduced. It is another object of the present invention to provide improved electron discharge tubeapparatus in which the draining of positiveions from the region occupied bythe electron stream is substantially prevented. i

A further object of the present invention is to provide improved electron beam devices in which the dispersion of the electron beam is greatly minimized 'by preventing the draining of posil tive lons from the region occupied bythebeam,

by means of an ion suppressor grid placednear the electron beam-'accelerating Agrid` and biased positively with respectto it.l l

A `further object of the present inventionv is to providean improved electron beam device in* which `the dispersion of `the `electron beam is greatly minimized by preventing the `draining of positive ions from the -regiontoccupied by `the beam,` by means ofsplitting a resonator positioned along the beam path `into two sections statically `insulated from each other and having `one section function as the accelerating grid and as the ion-suppressor electrode while being biased positively with respect to ,the` other section.- l

I Otherobjects and advantages of the present invention will become apparent from the follow-- ing specification and drawings, in which:

i Fig. r1 shows a schematic circuit diagramf one form of the present invention; i 1

Fig. 2 shows a schematic representation of second form of the presentinvention; and

Fig.` 3`shows aschematic representation of still another form ofA the present linvention.` Referring `to* Fig. 1, the present invention is disclosed as 'applied to a velocity modulationdevice of the type" shown in the above-mentioned patent.` However, it is to be understood that the present'invention` is equally applicable to any type of electron "beam device, butiis mostparticularly applicable to devices wherein an electron beam flows througha field-free* space, since it is in such field-free `spaces that the dispersion tendency of the beam can bemost readily corrected. W

In Fig.` 1 there is shown an electron'beam velocity modulation `device having a cathode schematically shown at Il, from which electrons are .projected through a hollow cavityresonator l2A through the entrance or accelerating grid i3I thereof, by means of accelerating battery I6. A focusing shield I4, as is well known in the art, may be placed around cathode II to direct electrons toward grid I3 from cathode II. Resonator I2 is also provided with an exit grid I'I through Which the electrons leave the space confined by the resonator I2 and enter the field-free drift space-:provided by conductingv drift tube I8; The electronsmay thereafterpass successively through the entrance and exit grids I9 and 20 of a second resonator 2I and are thereafter collected by a suitable collector 22. If desired, the vacuum envelope indicated at I8 may include resonators I2 and 2I and drift tube yIlas portions of the evacuated enclosure.

As described more in detail in the abovementioned Patent No. 2,242,275, if resonator I2 contains a high frequency oscillating electromagnetic field, the electron stream passingl therethrough will have the electrons thereof periodically and recurrently varied in velocity. These variable velocity or velocity-modulated electrons, upon passing within driftv tube` I8, become groupedbythe action of the faster electrons in overtaking the slower electrons. The grouped electron streampassing through the grids I9 and 2l! of resonator. 2i gives up energy at this high frequency to the eld within the resonator '2l from which this energy may be taken by suitable means (not shown) foruse as desired. By suitable connections, which are not material to the present invention and are not shown, the device just described may act as an amplifier, oscillator, detector, modulator, frequency multiplier, or in many other capacities. YThe velocity-modulated stream may be used in any desired way, so far asthe present invention is concerned, I

In devices of this type the path of the electron stream is relatively long, and the stream tends to become dispersed or defocused due to the mutualrepulsion of the negatively charged electrons comprising the stream. This action is frequently evident within drift tubes, such as I8, which define field-free spaces. where there .are substantially no outside potentials or fields acting onthe'stream. As a result, the electrons of the stream become quite dispersed before reaching grid- I9, andthe efficiency and rating or power capacity of the device are greatly decreased.

This dispersing action may be overcome bythe introduction or retention of a slight amountof gas or Avapor within the evacuated envelope Illof the device; so that the passage of electrons will ionze the material within the envelope I8. As

is well known, such ionization creates positively,V

charged ions. and negatively charged electrons. The electrons produced by ionization, having relatively large velocities with random directions, soon leave the region occupied by the electron beam. The positive ions, however, having relatively large mass and slow velocity, remain in this. region, and serve to neutralize the negative charges of the beam,v so that, as the electrons flow along the beam, their mutual. repulsion is neutralized and substantially no dispersion or defocusing occurs.

This phenomena has been utilized .in the prior art but has been found to be not fully satisfactory, in that dispersion ofthe electronbeam'is not completely overcome, especially where high velocity electron beams are used. AI have found that there is a Weak electrostatic field near-grid I3 on the sideeppositecathode Irl which attracts positive ions toward the cathode II'. This i'leld isproduced. by the arrangement of the electricI eld lines near the grid openings of grid I3 and may be considered to reach in through grid I3. This field draws positive ions toward it, leaving a nearby region of unneutralized electron beam which is negatively charged, due to the preponderance of negative charges in the beam. Therefore, positive ions are attracted from the next adjacent region, which then becomesnegative and in turn draws ions from ythe nextregion. This action continues and, in this way, the draining of positive ions from the region of the beam is extended well into drift tube I8. In order to reduce this effect, and according to the present invention, there is provided a further ion-suppressor or shielding grid 23 within the drift tube I8, close to grid I1 but insulated therefrom and maintained at zero or a slightly positive potential with respect thereto, as by means of battery 24. In other words, the potential gradient between suppressor grid 23 and portions of the drift tube I8 extending between the grid 23'and the second resonator 2| has its more positive portion at the grid 23; consequently, it can be said: with respect to the portions of the drift tube I8 extending between suppressor grid 23 and the second resonator 2|, the potential gradient has its more positive portion closer to the cathode' II. Since shielding grid 23 is now at a slightly positive potentialswith respect to the effective potential of the-beam, there will be notendency for positive ions'in drift tube I8 to be attracted to the cathode I`I or. grid I3, because of the shielding effect of grid 23. In this way, the major cause of the draining of positive ions from the electron beam is eliminated and the positive ions are left Within the =beam to neutralize the dispersion effectof the beam, so that the` beam has little or no tendency to spread. l

Instead of placing the` shielding grid 23 within the drifttube I8, itmay also be placed between resonator grid I3 and cathode II, in the manner shown at. 23 in Fig. 2. The battery 24 is `connected between the suppressor electrode 23' and the resonator I2; and the latter element in turn is electrically connected to the drift tube I8'so that the potential gradient between electrode 223 and drift tube I'8,- with respect to drift'tube I8, has its more positive portion closer tothe cathode II. This also has the effect of shieldingthe positive ,ions from the influence of the cathode II, and is even more effective than the arrangement shown in Fig. 1.

As anothermodication, the shielding electrode may beplaced at the far end of drift .tube I8, in. addition to or substitution-for electrode 23r shown in Fig. l, or grid 23 ofFig. 2, to prevent ion drain. ing at that end.

A further modification ofthe present invention is shown inlig.l 3, in'which, in place of 'utilizing a separate shielding'grid, the resonator I2is split into two sections. I2 and I2, relativelyinsulated from one another with respect to static potentials by any suitable insulation 25. In this-r instance, section I2 of resonator I2`. with vits'grid I3 asslunes the function of the shielding grids 23,23 `of the .prior figures. Thus, battery` 2.4' maintains section I 2' slightly.-v positive wi-thirespect. to section I2.\". .1 It will-be understood that the potential gradient betWeen-thesections I2 and I2, withrespect to section I`2f, thereby has its. more `positive portion closer to the cathode II'. In this way, there is no tendency zfor thepositive ions of the electron beam Within drift'tu-be I18fto be drainedout bythe action` of cathodefi'II.

If desired,.portion I2" may be insulated-fronti- 'alia-ateo Fdriftvtube I84 and portion: l2', and maintained positivewith respect thereto, whereby beam `disbeam tubes using elongated electron beams, of

the velocity modulation type or other type, and need not be restricted to the velocity-modulation type, or even' to the double-resonator velocity-modulation type. yAs many changes could be made in the above construction and many apparently widely different `embodiments of this `inventionA could be made without departing from the scope thereof, itis intended that all matter contained in the above descriptionor shown in the accompanying drawings shall be interpreted as illustrative and not in a, limiting sense.

Whatlis claimed is:

1. High frequency velocity modulation electron discharge tube apparatus, comprising means including a cathode for producing an electron beam, a cavity resonator positioned along the path of said beam and adapted to contain a confined electromagnetic eld for interaction with said beam, drift-space means beyond` said resonator` defining a field-free drift space surrounding the path of said beam beyond said resonator, said beam path being evacuated except for residual gas, and positive-ion-repelling means along said path for producing a potential gradient having its more positive portion closer to said cathode, thereby shielding said drift space from said cathode to prevent draining of the positive ions of said gas from said drift space whereby dispersion of said electron beam is minimized.

2. High frequency velocity modulation electron discharge tube apparatus as in claim 1, wherein said last-named means -comprises a shielding electrode, and means coupled to said electrode for applying a potential to said electrode positive with respect to said drift-space-deiining means.

3. High frequency velocity modulation electron discharge tube apparatus as in claim 1, wherein said last-named means comprises a shielding electrode positioned between said beam-producing means and said resonator.

4. High frequency velocity modulation electron discharge tube apparatus as in claim 1, wherein said resonator comprises two insulated portions and wherein said shielding means comprises means for maintaining one of said portions at a different static potential from the other of said portions.

5. High frequency velocity modulation electron discharge tube apparatus as in claim 1, wherein said resonator comprises two insulated portions and wherein said shielding means comprises means for maintaining the one of said portions adjacent said drift space at a positive potential with respect to the other of said portions.

6. High frequency velocity modulation electron discharge tube apparatus comprising `means including a cathode for producing an electron beam, a cavity resonator positioned along the path of said beam and adapted to contain a coniiined electromagnetic eld for interaction with said beam, drift-space means beyond said resonator defining a field-free drift space surrounding the path of said beam beyond said resonator, said beam path being evacuated except for residual gas, and means along said path for shielding said drift Space from said cathode to prevent draining of `the positiveionsof said gas from said drift space, whereby dispersion of said electron beam Vis minimized, said` last-named N.means comprising a shielding electrode within said drift space, and means connected to said electrode for positively charging said electrode with respect to said driftspace-dening means. f

7. High frequency velocity modulationelectron discharge tube apparatus comprising means including a cathode for producing an electron beam, a cavity resonator "positioned along the path of said beam and adapted to contain a confined elec- 4tromagnetic eld for interaction with said beam, drift-space means bey-ond said resonator deining a field-free drift space surrounding the path of said beam beyond said resonator, said beam'path being evacuated except ioriresiduai gas; and means along said path `for shielding said drift space fromsaid cathode to prevent draining of the positive ions of said gas from said drift space, whereby dispersion of said electron beam is minimized, said last-named means comprising a shielding electrode within said drift space and adjacent said resonator, and means connected to said electrode for applying a positive potential to said electrode with respect to said drift-spacedeiining means.

8. High frequency electron beam apparatus comprising means for producing an electron beam, conductive means along the path of said beam and defining a field-free drift-space enclosing said path, said beam path being evacuated except for residual gas, and a shielding electrode positioned within said drift space and having a positive potential with respect to said conductive means, whereby the draining of the positive ions of said gas from said drift space and dispersion of said beam is minimized.

9. High frequency electron beam apparatus comprising conductive means defining a field-free drift space, means aligned with said first means for projecting a beam of` electrons through said drift space, a shielding electrode located within said drift space near the point of entrance of said electrons, and means coupled to said electrode for maintaining said electrode at a positive potential with respect to said conductive means, whereby the draining of positive ions from said drift space and dispersion of said beam is minimized.

10. High frequency electron beam apparatus compirsing conductive means deiining a fieldfree drift space, means aligned with said rst means for projecting an electron beam through said drift space, and means comprising a shielding electrode located within said drift space for preventing draining of positive ions from said drift space, whereby dispersion of said beam is minimized.

11. High frequency electron beam apparatus comprising means for producing an electron stream, a cavity resonator positioned along the path of said stream for interaction therewith, an envelope surrounding the path of said stream and evacuated except for residual gas, conductive means defining a field-free drift space surrounding said stream path, an electron-permeable electrode mounted across the end of said drift-spacedening means opposite said cathode, and positive-ion-repelling means along said path for producing a potential gradient having its more positive portion closer to said cathode, thereby preventing passage of positive ions from said drift space toward said cathode.

12. High frequency electron beam apparatus as inclaim 11, wherein said last-named means comprises aneiectron-permeable suppressor electrode .between said cathode and said first-named elec- ,trodegv and means connected to said suppressor :electrode formaintaining said suppressor electrede; at a positive potential relative to said firstnamed` electrode.

13. High frequeny electron beam apparatus comprising Ymeans for producing an electron stream, a cavityv resonatorpositioned along the pathof said stream for interaction therewith, an y1 adjacent said rst-named'electrode, and'me'ans.

coupled to said suppressor electrode for main,- taining said suppressor electrode ata positive po.- tential relative to saidfirst-namedelectrode;

5 RUSSELL I-I. VARIAN.

REFERENCES CITED The following references are of record inthe o ie of this patent: i

UNITED STATES PATENTS` Number y Name Y Date 1,603,284 Johnson Oct; 19, 1926 1,632,080 Johnson June 14,1927

15 2,263,184A Mourontseff et a1. Nov. 18, 1941 2,272,165- Varian et al f Feb. 3, 1942 2,275,480 Varian etal Marx-10, 1942 2,280,026 Brown' Apr. 14, 1942 2,305,344 Clark Dec, 22, 1942 Motz Ju'ly s, 1943 

